In recent years, biodegradable plastics have been actively developed as materials that can solve problems caused by plastic waste that places a heavy burden on the global environment, such as impact on the ecosystem, generation of harmful gases during combustion, and global warming due to a large amount of heat generated by combustion.
In particular, carbon dioxide generated by combustion of plant-derived biodegradable plastics was originally present in the air, and therefore the amount of carbon dioxide in the air does not increase. This is referred to as carbon neutral, and is regarded as important under the Kyoto Protocol that sets carbon dioxide reduction targets. For this reason, biodegradable plastics have been expected to be actively used.
Recently, from the viewpoint of biodegradability and carbon neutral, aliphatic polyester resins, especially polyhydroxyalkanoate (hereinafter, sometimes referred to as PHA) resins have received attention as plant-derived plastics. Among PHA resins, for example, poly(3-hydroxybutyrate) homopolymer resins, poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymer resins, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) copolymer resins (hereinafter, sometimes referred to as P3HB3HH), poly(3-hydroxybutyrate-co-4-hydroxybutyrate) copolymer resins, and polylactic acid have received attention.
However, the PHA resin is low in crystallization speed. Therefore, the PHA resin requires a long cooling time for solidification after heat melting in molding processing. Thus, productivity is poor. On this account, a spinning velocity when melt-spinning the PHA resin needs to be extremely low, so that there are restrictions from a viewpoint of practical use. Further, to increase the strength, the PHA resin needs to be stretched after the take-up.
Disclosed as a prior example of a technology of melt-spinning the 3-hydroxyalkanoate polymer is a cold stretching method of: immediately after the P3HB3HH is extruded from a melt extruder, rapidly cooling the P3HB3HH to not more than a glass transition temperature Tg of the resin to prevent blocking of filaments of the P3HB3HH; and quickly and partially crystallizing the P3HB3HH at a temperature not lower than the glass transition temperature Tg (see PTL 1). This method can realize the spinning of a polymer, such as the P3HB3HH, which is hardly crystallized, and can produce stretched filaments having unique properties. On the other hand, since this method requires an essential step of rapidly cooling the P3HB3HH to not more than the temperature Tg (about 0 to 4° C.) immediately after the spinning, energy consumption is large, and large-scale equipment is required. Thus, problems remain from a viewpoint of practical use.
On the other hand, it is known that by melt-spinning thermoplastic polyesters such as poly(ethylene terephthalate) or poly(butylene terephthalate) at a high spinning velocity, fibers having properties that are sufficient from a viewpoint of practical use are obtained without mechanically stretching the fibers. Prior examples of such high-speed spinning method have been disclosed (see PTLs 2 to 11). However, the 3-hydroxyalkanoate polymer which is different in crystallizability from the thermoplastic polyester has not been disclosed.
Further, disclosed as another prior example is a producing method of spinning hollow cross section yarn or multi-lobe cross section yarn of biodegradable aliphatic polyester at high speed while limiting a melt flow rate and a spinning temperature (see PTL 12). A molecular structure, such as a copolymerization ratio, of the 3-hydroxyalkanoate polymer significantly influences on the crystallizability and spinnability of the 3-hydroxyalkanoate polymer, and the strength of the obtained fiber. However, this prior example does not disclose or suggest an appropriate copolymerization ratio.
Furthermore, disclosed as yet another prior example is a producing method of melt-spinning a polylactic acid-polyethylene glycol copolymer at a speed of not less than 4,000 m/min (see PTL 13). A high-speed spinnability of this method is higher than that of a producing method of melt-spinning a polylactic acid alone. However, it is thought that since the polylactic acid that is easily hydrolyzed is copolymerized with a hydrophilic polyethylene glycol block, the polylactic acid-polyethylene glycol copolymer is more easily hydrolyzed. Thus, a problem is that the management of the amount of water absorption is difficult.